The invention concerns a drilling system having a drilling head fixed to a drill string which comprises an outer pipe and a percussion string inserted therein, wherein the percussion string comprises a plurality of rods which bear against each other with their end faces.
A drilling system of that kind is known from EP 0 387 218 B1. This involves a rock drilling arrangement for producing straight boreholes for receiving anchors for buildings or explosive charges for carrying out rock blasting operations. In that case the cylindrical shank of the drilling bit is mounted axially displaceably to the front end of the outer pipe by way of a cylindrical guide which is several centimetres long and which is in contact with a small clearance. The same applies in regard to the free end of the rear drill rod against which a hammer or percussion piston strikes to apply the percussion forces. Each individual rod is guided in the region of two bushes at two positions on its length. Provided in the region of the guides for the percussion rod are axially extending ducts for passing therethrough a flushing medium, which make it possible for a flushing medium to be conveyed towards the drilling head from the rear end of the drill string through the intermediate space between the outer pipe and the percussion string or through the axially extending ducts between the outer pipe and the percussion string. The end faces of the individual rods of the percussion string, which bear against each other, extend in the radial direction so as to afford a maximum effective surface area for transmission of the axially acting percussion forces.
The arrangement described in EP 0 387 218 B1 has some major advantages which are essentially that the inner percussion string comprises various individual rods which bear against each other without screwing. The individual short rod has a natural frequency which is very much higher than a long screwed percussion string. Thus, in terms of transmission of the percussion force, very much harder and undamped transmission of the percussion force is afforded by way of a plurality of short rods which bear against each other without a screw connection. Added to that is greater ease of handling during the drilling operation. After the drilling arrangement is advanced by the length of an outer pipe section or an inner rod, the rotary and percussion drive is separated from the drill string and a fresh inner rod and a fresh outer pipe is introduced into the drill string. That situation involves time savings by virtue of the fact that the inner rod to be inserted does not have to be screwed in place.
The arrangement known from the above-quoted document is however suitable by virtue of its structure only for making bores which extend precisely in the axial direction of the drill string.
The object of the present invention is to provide a drilling system which permits a greater variation in the drilling direction.
In accordance with the invention that object is attained in that the outer pipe is adapted to be deformable along its longitudinal axis and the end faces of two rods which bear against each other are such that they bear against each other substantially in surface contact upon inclined positioning of the axes of the two rods relative to each other.
Drilling systems with elastically bendable outer pipesxe2x80x94so-called directional drilling systemsxe2x80x94are known from the state of the art, for example from DE 196 12 902 A1. That publication states that a drill string having a drilling head which produces a curved borehole configuration is used for directional drilling. In straight-line drilling the drilling head is rotated at a uniform, generally low angular speed so that the force deflecting the drilling head is uniformly distributed to the entire periphery of the drilling head and is thus cancelled out. For drilling a radius, the drilling head remains in a given angular position without drilling drive so that it follows the curved path which is predetermined by virtue of its structural features. In that case the drilling heads may be of very different configurations. The drill string is usually mounted on a rail-guided sliding carriage connected to a linear drive and has a rotary or rotary-percussion drive with which the string can be caused to rotate and possibly also driven into the ground. In the previously known directional drilling systems the outer string was in principle used for transmission of the percussion force. Besides the above-described problem that the long outer string has a low natural frequency and is of a high mass, that gave rise to an additional problem that the wall friction of the outer string which is guided in the curvedly extending borehole in the earth nullifies a considerable proportion of the percussion energy. Furthermore, in addition to the mass of the outer pipe, the mass of the flushing medium contained in the outer pipe also has to be accelerated by the percussion drive. Finally, a hammer blow on the rear end of a curved pipe produces not only axial acceleration but also a bending force. In practice it has been found that the percussion force acting on the rear end of the drill string scarcely arrives in the region of the drilling head.
The inner string which can be found for example in FIGS. 6 and 7 of DE 196 12 902 A1 could not be used for percussion force transmission purposes. Either it was proposed that the individual elements of the inner string are connected together by way of universal joints which are destroyed by ongoing percussion forces. Alternatively, it was proposed that the universal joints be omitted, if the inner string is sufficiently flexible. With a high degree of flexibility however, it is not possible to achieve a sufficiently great percussion force transmission effect.
The proposal in accordance with the present invention, to provide a drilling system with rods which bear against each other in unscrewed relationship as a directional drilling system with a flexible outer pipe permits the transmission of percussion force by way of the inner percussion string if the end faces of two rods, which bear against each other, are so designed that they bear against each other substantially in surface contact even upon inclined positioning of the axes of the two rods. In other words, based on the drilling system described in the opening part of this specification and disclosed in EP 0 387 218 B1, end faces which depart from the flat radial shape had to be proposed, so as to ensure effective transmission of percussion forces even in a situation involving bending of the outer pipe which results in inclined positioning of the longitudinal axes of two drill rods relative to each other.
In comparison with the previously known transmission of percussion forces in directional drilling systems by way of the outer pipe, percussion force transmission by way of an inner percussion string has the crucial advantage that the percussion force cannot be reduced by virtue of friction of the percussion string against the wall of the borehole. As a general rule a flushing medium is passed between the outer pipe and the inner string, the flushing medium comprising for example water with swellable clay (bentonite). The aqueous swellable clay is of a viscous to pasty consistency and produces relatively slight frictional resistances upon movement of the percussion string with respect to the outer pipe. In that respect the flushing medium itself is not accelerated by the hammer blows and cannot absorb any percussion energy.
The hammer blows are transmitted by short straight rod sections of the inner string, in which respect no bending forces can occur as the individual rods of the inner string are not curved.
An essential feature of the invention provides that, in the case of the inner percussion string of the directional drilling system according to the invention, no fixed connection exists between the ends of the individual rods of the percussion string. In particular, screwing of the rod ends was eliminated. A screwed percussion string is unsuitable precisely in relation to directional drilling in whichxe2x80x94unlike the situation with straight drilling operationsxe2x80x94often only a slow rotary drive for the drilling head is involved or the drilling head remains completely in a specific angular position for a relatively long period of time. If a permanent hydraulic percussion drive acts on a screwed string, the screw connections generally loosen due to the hammer blows. It is only if the string is constantly driven in the fastening direction of the screw means by a rotary drive that it is ensured that the screw connections do not come apart, in spite of the hammer blows on the string. In the case of a directional drilling arrangement in which the rotary drive often has to be stopped for a relatively long period of time, there is the risk that the screw connections of the individual rods of the percussion drive come loose because of the hammer blows, and that results in destruction of the percussion string upon further forward drive movement of the drilling system.
That risk does not occur in the drilling system according to the invention which eliminates fixed connections between the rod ends and in particular screw connections between the rod ends.
As the outer pipe is adapted to be deformable along its longitudinal axis, that is to say the longitudinal axis is bendable in a radius about a centre of a circle, care should be taken to ensure that each rod is supported against the inner wall of the outer pipe only in one or two short regions of the length of the rod. In that respect, the preferred structure is one in which each rod is supported against the outer pipe only in a single annular region of the rod periphery and in the other regions of its length it is of an outside diameter which is one or more centimetres smaller than the inside diameter of the outer pipe. In the region of a bend in the outer pipe the inner percussion string can extend from one support location to another in various straight sections.
Care should still be taken to ensure that flushing medium can pass unimpededly through the annular space between the outer pipe and the percussion string. For that reason, in the region in which each rod of the percussion string is guided against the inner wall of the outer pipe, there should be provided a recess which extends in the axial direction or a duct which extends in the axial direction, so that the flushing medium can still pass therethrough. For example, grooves which extend in the longitudinal direction and through which the flushing medium flows can be provided in the wide regions of the rod, which bear against the inner wall of the outer pipe. Alternatively, the outer pipe can be provided over its entire length with axial grooves for the flushing liquid to be passed therethrough. That means however that it is necessary to reckon on an increase in the manufacturing costs for the outer pipe.
In a particularly preferred embodiment of the invention the end faces, which bear against each other, of two rods of the percussion string are curved on the one hand convexly and on the other hand concavely. Preferably each rod of the percussion string has a first end with a ball head and a second end with a ball socket, wherein the radii of curvature of the ball surfaces of the ball head and the ball socket substantially correspond to each other. The percussion rod of the percussion drive, on which the percussion piston of the percussion drive acts, should then have a surface which is complementary to the end face of the rearmost rod of the percussion string. Likewise the shank of the drilling bit with the drilling head has an end face which is complementary to the foremost end face of the foremost rod of the percussion string.
When the end of the percussion rod is in the form of a ball head, the ball head preferably forms the region for radial support of the rod against the inner wall of the outer pipe. The section of the rod, which extends from the ball head and which is in the form of a cylindrical rod, is of a smaller diameter than the ball head. To form the axially extending flow ducts for the flushing medium, the ball head has axially extending recesses which are arranged in the region of its equator, with respect to the longitudinal axis of the rod.
As mentioned in the opening part of this specification, a rotary force is transmitted to the drilling head in order either to rotate it continuously or to move it into a given angular position when a radius is to be drilled. In the case of directional drilling systems in accordance with the state of the art, in which percussion forces which are possibly produced are transmitted by way of the outer pipe, the drilling head is simply rigidly connected to the outer pipe. In the present case in which percussion forces are transmitted to a drilling bit, the drilling bit can be held non-rotatably in the outer pipe, in which case it should be movable axially by a certain distance. The axially movable support for the drilling bit ensures that the percussion energy acting on the drilling bit is not applied to the outer pipe. The drilling bit is displaceable with respect to the outer pipe so that the percussion energy is transmitted directly on to the bottom of the borehole by way of the drilling head.
The non-rotatable fitment of the drilling bit in the outer pipe can be achieved for example by a positively locking connection between the shank of the drilling bit and the outer pipe. The shank of the drilling bit can be provided with an external spline or tooth configuration which engages into an internal spline or tooth configuration of the outer pipe. The rotary drive is then connected to the rear end of the outer pipe and is preferably hydraulically actuated to achieve the required torque levels.
Alternatively the torques can be transmitted to the drilling head by way of the percussion string if the ends of two rods which bear against each other have connecting elements which engage into each other in positively locking relationship. For example, one of the ends, in particular the end in the form of a ball socket, can be provided with a recess into which projects a projection at the other end, in particular the end in the form of the ball head. The ball socket, in the region of the outer periphery of the ball, may have a groove disposed on a great circle extending in the longitudinal direction of the rod. The ball head, at two mutually diametrally oppositely disposed positions, may have a respective cylindrical protrusion, each of the protrusions engaging into an end of the groove in the ball socket. The protrusions can be displaced in the direction of the groove and pivoted about their protrusion axis. Such a claw-like connection between the end of the first rod and the end which bears thereagainst of the second rod permits the transmission of sufficiently high rotary forces. In an embodiment of that kind, the drilling bit must also be non-rotatably connected to the foremost end face of the percussion string. The rear end of the percussion string in that case must be non-rotatably connected to the rotary drive so that rotary forces can be transmitted from the drive unit outside the borehole to the drilling head. The frictional loss can also be considerably reduced by virtue of transmission of the rotary forces by way of the inner percussion string. The rotary forces do not have to be transmitted against the friction within the entire borehole, but only against the frictional forces operative between the outer pipe and the percussion string.
The above-described claw-like connection between the rod ends only represents an example. Any other connections involving a positively locking relationship which permit pivotal movement of the individual rods of the percussion string relative to each other are possible. In that respect, it is to be noted that a motion play of a few degrees between the two end faces of the rods may be sufficient to permit the required inclined positioning between two rods. By virtue of the limited flexibility of the outer pipe, in general very large radii for the borehole axis are achieved, so that the individual rods are each inclined relative to each other only by a few degrees.
Preferably the ends of two rods which bear against each other have guide elements which guide the protrusion for the transmission of rotary force into the recess, when the rod ends bear and press axially against each other. That ensures that for example when fitting a new outer pipe and a new inner rod to the drill string, the non-rotatable connection between the individual rods of the drill string is achieved without involving special adjustment by the operators. Even if the rods of the inner string come loose from each other when inserting a new section of the drill string, the non-rotatable connection between the individual rods is restored automatically by virtue of the action of the guide elements, when the drill string is subsequently fixedly connected to the drive unit.
In this embodiment the rotary drive has to be connected to the percussion string. In order not to apply percussion forces to the rotary drive or the transmission assembly of the rotary drive, a percussion rod should be held axially movably but non-rotatably in the rotary drive. For that purpose a drive pinion may have an internal tooth configuration which co-operates with an axially extending external tooth configuration on the percussion rod and which ensures freedom of axial movement with a positively locking connection in the peripheral direction.
A seal is preferably arranged between the shank of the drilling bit and the outer pipe to prevent uncontrolled discharge of the flushing liquid. The shank of the drilling bit also has an axially extending duct through which the flushing liquid or the flushing medium is passed from the annular space between the percussion string and the outer pipe to the drilling head.
In order to fix the drilling bit within the end section of the outer pipe, the outer pipe, near the drilling head, has a radial reduction in inside diameter, while arranged on the shank of the drilling bit is an enlargement in diameter, which is greater than the reduction in the inside diameter of the outer pipe. In that way the drilling bit is secured by the radial diametral enlargement to prevent it from falling out of the end section of the outer pipe. In a practical embodiment the entrainment profile of the outer pipe in the form of an internal spline or tooth configuration is screwed fast to the end of the outer pipe. That screw connection preferably fixes a divided holding ring which can be fitted into the outer pipe and which forms the reduction in the inside diameter of the outer pipe. Also mounted on the shank of the drilling bit is an annular body which forms the enlargement in the diameter thereof. The element with the spline configuration, which is screwed to the end of the front section of the outer pipe, preferably also carries a sensor or signal generator, by means of which it is possible to ascertain the position of the drilling head by way of a measuring device outside the borehole so that the drilling drive can be controlled to achieve the desired drilling configuration.
Preferably all sections of the outer pipe are connected together by screw sleeves. The screw sleeves may be of a diameter which is enlarged with respect to the diameter of the sections of the outer pipe, for receiving the ball head.
The percussion drive for the percussion string strikes against the rod of the percussion string, which is rearmost in the direction of advance movement. It is generally flange-mounted behind the rotary drive, in which case it acts on a percussion rod which protrudes through the rotary drive and which is axially displaceable with respect to the rotary drive so that the percussion forces applied thereto are not transmitted to the rotary drive but to the percussion string.
The feed for the flushing liquid is preferably arranged near the front end of the percussion rod at a screw connection between the rotary drive and the rearmost section of the outer pipe and is formed by a radial duct which acts through the outer pipe into the annular space between the outer pipe and the percussion string. Preferably, arranged between the outer pipe and the front section of the percussion rod is a seal set which seals off the annular space between the outer pipe and the percussion rod. That ensures that the flushing medium is conveyed exclusively through the annular space between the outer pipe and the percussion string forwardly to the drilling head and not rearwardly in the direction of the drive for the drill string.
In a further preferred embodiment the percussion string can be arrested selectively in the axial direction with respect to the outer pipe. The arresting effect operates at least in the forward feed direction in which the percussion forces also act. The arresting means provide that the percussion forces are transmitted from the piston by way of the percussion string to the outer pipe. As long as the percussion forces are to be used to drive the drilling bit forwardly as rapidly as possible, the outer pipe is to be uncoupled from the percussion string so that the percussion forces act exclusively on the drilling bit and are transmitted thereby to the bottom of the borehole. If however there is a wish to apply hammer blows to the outer pipe by way of the percussion mechanism, for example in order to overcome high frictional forces in the borehole, the outer pipe can be coupled to the percussion string. The percussion forces can also be temporarily applied to the outer pipe which comprises a plurality of pipe sections screwed together, in order to release the screw connections between the pipe sections. The coupling, that is to say the connection which is fixed in the axial direction, must be ensured at least in the direction in which the percussion forces act.
Preferably, the coupling between the outer pipe and the percussion string is effected in the region of the drilling bit at the front end of the drill string. In that way, the drill string is subjected to a pressure loading by the percussion mechanism and transmits its pressure forces at the front end in the region of the drilling bit to the outer pipe. The latter is pulled by the percussion forces in the forward feed direction or the percussion direction. Preferably, the enlargement in diameter of the drilling bit, which fixes it in the outer pipe, is used to provide for axial coupling. For that purpose, the enlargement in diameter can be adapted to be arrested in the condition of bearing in the axial direction against the reduction in diameter of the outer pipe.
That can be achieved by the outer pipe being mounted to the forward drive machine displaceably in the axial direction and fixably in at least two different axial positions. For example, a part of the outer pipe may have radial pins or protrusions which are guided in a sliding sleeve which is fixed to the forward drive machine. For each radial protrusion the sliding sleeve has a guide groove with an axial portion and two holding portions extending in the peripheral direction at the two ends of the axial portion. The radial protrusions of the outer pipe can be accommodated in the guide groove either in the first holding portion or in the second holding portion. In the first holding portion the front end of the front pipe end section of the outer pipe bears against the rearward contact face of the drilling bit so that the drilling bit is freely held in the outer pipe in the forward direction, that is to say in the percussion and forward feed direction. In contrast, in the second holding portion, the reduction in diameter of the outer pipe bears against the enlargement in diameter of the drilling bit so that the axial percussion forces are transmitted to the outer pipe by way of the drilling bit.